This invention relates to regulation of the raster size of a television receiver.
The accelerating potentials, such as ultor voltages for the electron beams of a cathode ray tube of a television receiver, are obtained from the high voltage circuits of the horizontal output transformer. The width of each horizontal raster line will vary with variations in accelerating potential, the width increasing with decreasing ultor voltage. This effect is commonly called blooming. Decreasing ultor voltage can be caused by heavy beam current loading of the high voltage circuits of the output transformer.
Raster line width is also determined by the magnitude of the voltage applied across the horizontal deflection winding during the trace interval of the deflection cycle, the width of the raster line decreasing with decreasing voltage applied across the winding. The voltage across the winding is frequently derived from a regulated B+ voltage coupled to the horizontal deflection circuit. Heavy video loading will result in an increased load current being drawn from the B+ voltage source by the horizontal deflection circuit.
To compensate for the effects of blooming, prior art circuits, such as disclosed in U.S. Pat. No. 3,444,426, granted to M. E. Buechel, have placed a resistance in series with the load. The increased voltage drop across the resistance decreases the voltage across the deflection winding, decreasing the raster line width. Such raster line width regulation, however, undesirably increases power dissipation. The aforementioned patent also includes a series pass transistor in the connection between the B+ supply and the deflection circuit that functions primarily to introduce a parabolic waveform for pincushion distortion correction.
Other circuits, such as disclosed in U.S. Pat. No. 3,626,238, granted to G. Forster, have used 60 cycle SCR phase controlled voltage regulators for providing regulated B+ voltage. An RC filter arrangement, in order to eliminate the 60 Hz ripple, places relatively large valued resistors in series with the horizontal deflection circuit. When heavy load current is drawn, the voltage coupled to the horizontal deflection winding, because of the IR voltage drop across the resistors, decreases much more than is needed to compensate for blooming. A negative feedback loop senses the voltage drop and increases the conduction angle of the SCR to increase the B+ voltage, thereby obtaining the correct raster width. As with the first mentioned prior art circuits, the correct voltage needed to prevent blooming is obtained by decreasing the voltage applied to the deflection winding through use of resistive voltage drops, thereby undesirably increasing the power dissipation.